The instruments that meaasures rotational speeds of motors and generators are called the tachometers.
Following are the different kinds of tachometers:
Following are the tachometer generators that are termed below:
In a D.C. generator the e.m. fen gendered depends on the following factors:
The permanent magnet pole pieces that are used in the field system. It produces a certain amount of voltage that depends on the speed. Hence the emf generated can be easily tracked by the speed.
A D.C. tachometer generator is illustrated in Fig. 32 given below:
The illustrated model consists of:
So the voltage is proportional to speed, and it can be standardized only with speed (r.p.m.)
Due to certain disadvantages of the D.C. tachometer generator, the A.C. tachometer generator was being created. The commutator and brushes are removed in A.C. tachometer generator, and this makes it different from the D.C. generator.
An A.C. tachometer generator is diagrammatically represented in Fig. 33 given below.
The above-given figure consists of the following features:
Therefore to measure speed either the amplitude of the voltage or the frequency or both can be utilized. In an A.C. tachometer, the induced voltage is thus the desired parameter.
Fig. 34 thus represents the photoelectric tachometer.
But when the opaque part comes in between the light source and the sensor then the light gets hindered resulting in no pulse output. Therefore it can be concluded that whenever a hole comes in line with the light source and sensor, only then a pulse is thus generated. These pulses are then measured through an electronic device.
The number of pulses depends on:
However, as a matter of fact,the number of holes in the disc are fixed, so the amount of pulse created depends on upon the speed of the shaft. The electronic device that counts these pulses is standardized by (r.p.m.)
Advantage. It is a digital instrument.
Disadvantage. The light source has to change periodically, and if the grating period is made least,then there is a chance of getting erroneous output.
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